The fluid catalytic cracking (FCC) process has become well-established in the petroleum refining industry for converting higher boiling petroleum fractions into lower boiling products, especially gasoline. In fact, the fluid catalytic cracking process has become the preeminent cracking process in the industry for this purpose, providing approximately 95% of the total catalytic cracking capacity in the United States.
In the fluid catalytic process, a finely divided solid cracking catalyst is used to promote the cracking reactions which take place in the feed. The catalyst is used in a very finely divided form, typically with a particle size range of 20-300 microns, with an average of about 60-75 microns, in which it can be handled like a fluid (hence the designation FCC) and in this form it is circulated in a closed cycle between a cracking zone and a separate regeneration zone. In the cracking zone, hot catalyst is brought into contact with the feed so as to effect the desired cracking reactions after which the catalyst is separated from the cracking products which are removed from the cracking reactor to the associated fractionation equipment for separation and further processing. During the cracking reaction, coke is deposited on the catalyst and this deposit of coke deactivates the catalyst so that it needs to be regenerated before it can be reused. Fortunately, the coke deposit can be made to serve a useful purpose. Cracking is an endothermic reaction which requires the input of significant amounts of heat in order to carry the cracking reactions to completion. Although, in principle, heat could be supplied by raising the temperature of the hydrocarbon feed prior to contact with the catalyst, this would result in a significant degree of unselective thermal cracking so that very little control could be effected over the product distribution and, in addition, considerable coking would occur in the furnace and other equipment used for heating and conveying the feed to the cracker. For this reason, it is generally preferred to supply the heat to the cracking reaction by means of the catalyst although the feed may be preheated to a certain degree in order to maintain an appropriate heat balance in the cycle. Heat for the process is supplied by the regeneration step in the cycle in which the spent catalyst is subjected to oxidative regeneration so as to remove the carbon deposits accummulated during the cracking step and to supply heat to the catalyst by the exothermic oxidation reactions which take place during regeneration.
The regeneration takes place in a separate regenerator vessel in which the catalyst is maintained in a fluidized bed into which an oxygen-containing gas, usually air, is admitted through a distribution grid which is designed to provide efficient mixing of air with the spent, coked catalyst. During the regeneration step, the coke on the spent catalyst is oxidized and the heat from the oxidation is transferred to the catalyst to raise its temperature to the requisite level for continuing the cracking reactions. The hot, freshly-regenerated catalyst is then returned to the cracking zone for contact with further feed together with any recycle. Thus, the catalyst circulates continuously in a closed cycle between the cracking zone and the regenerating zone with heat for the endothermic cracking reactions being supplied in the regenerator by oxidative removal of the coke deposits which are laid down during the cracking portion of the cycle. In order to maintain the desired level of catalyst activity and selectivity, a portion of the circulating inventory of catalyst may be withdrawn intermittently or continuously with fresh, make-up catalyst being added to compensate for the withdrawn catalyst and the catalyst losses which occur through attrition and loss of catalyst from the system.
A further description of the catalytic cracking process and the role of regeneration may be found in the monograph, "Fluid Catalytic Cracking With Zeolite Catalysts", Venuto and Habib, Marcel Dekker, New York, 1978. Reference is particularly made to pages 16-18, describing the operation of the regenerator and the flue gas circuit.
The amorphous cracking catalysts which were initially used in the FCC process were characteristically low activity catalysts which gave a relatively low hydrocarbon conversion with a relatively low carbon lay-down on the catalyst. Because the carbon provides the heat for the regeneration process, the carbon lay-down is a measure of the heat which can be produced during the regeneration and, consequently, of the regeneration temperature. Thus, the use of amorphous catalysts implied the use of relatively low regeneration temperatures.
The development of synthetic zeolite cracking catalysts, especially the zeolite cracking catalysts represented mainly by the synthetic faujasite zeolite Y, typically in the form of rare earth exchanged zeolite Y (REY) or ultrastable Y (USY) represented a considerable advance in the technology of the FCC process, but it was accompanied by its own problems. In contrast to the older, amorphous cracking catalysts which they rapidly supplanted, the zeolite catalysts were characterised as relatively high conversion catalysts which produced a relatively high carbon lay-down on the catalyst. The relatively higher carbon lay-down resulted in higher regenerator temperatures and higher burning rates both for the carbon on the catalyst and for the carbon monoxide produced during the combustion process. With the production of greater heat in the regenerator, the catalyst circulation rate was reduced since the process as a whole needs to remain in a heat balanced condition and this was desirable since it enabled the catalyst make-up rate to be reduced, a valuable economic factor.
The zeolite cracking catalysts are, in general terms, more sensitive to residual carbon than the amorphous catalysts, particularly with respect to selectivity. This sensitivity, coupled with the fact that operation under high temperature regeneration conditions was desirable for other reasons, as indicated above, provided an incentive for higher regenerator temperatures and lower residual carbon levels on the regenerated catalyst. To achieve this, it became necessary to carry the combustion of the carbon in the regenerator through to the final oxidation product, carbon dioxide, rather than to stop at the intermediate stage of carbon monoxide with the highly exothermic combustion of this product taking place in an external CO boiler. With operation in the full CO burning mode, the residual carbon levels on the catalyst fell to about 0.25 wt. percent or lower as compared to about 0.5 wt. percent with the amorphous oxide and clay catalysts operating in a non-CO burning regenerator. According to present standards a residual coke content of 0.1% or less coke on the regenerated catalyst is considered to represent a "clean burned" catalyst with higher coke contents being representative of only partial regeneration.
Two types of FCC regenerators are now in general use. The first type is the high-inventory, dense bed type regenerator in which the catalyst enters the regenerator from the reactor and combustion is induced in a dense bed of catalyst by means of combustion air which is injected from below. Typical regenerators of this type are shown, for example, in 1984 Refining Process Handbook, "Hydrocarbon Processing", Sept., 1984, pp. 108-109, Venuto and Habib, op cit p. 17 and U.S. Pat. Nos. 4,072,600 and 4,300,997. In regenerators of this type, a dense bed of catalyst is fluidized by the injected air with a dilute phase of catalyst above the dense bed. In one variant of this type of regenerator, the spent catalyst from the reactor is introduced tangentially into the dense bed of catalyst to impart a swirling motion to the dense bed so that the catalyst moves in an approximately circular path during regeneration until it is removed through a discharge port below the top of the dense bed. In order to ensure a sufficiently long average residence time in the regenerator, the discharge port is positioned around the vertical axis of the regenerator from the catalyst inlet port. An angular separation of 270.degree. measured in the direction of swirl is typical. The relatively high volume of catalyst in the regenerator ensures that the average residence is longer than that required to make one circuit of the regenerator so that some of the hot, regenerated catalyst will continue in the swirling bed to mingle with the spent catalyst from the reactor so as to promote the combustion of the coke deposits on the newly added spent catalyst.
The gases passing through and rising from the dense bed carry with them a portion of the catalyst from the dense bed and this produces a dilute phase of suspended catalyst above the dense bed. The dilute phase passes to cyclone separators at the top of the regenerator vessel where the catalyst is removed, to be returned to the dense bed through diplegs attached to the cyclones. The effluent gases from the regenerator are then discharged from the unit.
When this type regenerator is operated in the CO combustion mode, a CO combustion promoter such as platinum or another noble metal is conventionally provided in the catalyst, either as a component of the catalyst itself or, more usually, as an additive. The use of CO combustion promoters is described in U.S. Pat. No. 4,072,600. The effluent gases contain relatively higher amounts of carbon dioxide so that the CO boiler no longer serves any purpose, although waste heat recuperators are typically provided to recover the sensible heat of the regenerator effluent gases. With essentially complete combustion of the carbon monoxide in the regenerator, substantially all of the heat which is potentially recoverable from the coke deposited on the catalyst is returned to the catalyst with the result that the unit operates in a thermally balanced mode with minimal feed preheat requirements, as compared to the earlier mode of operation in which significant quantities of heat were lost in the downstream CO boiler.
An associated development in the technology of regeneration is represented in the low-inventory type regenerator in which combustion of the coke on the spent catalyst is initiated and carried through to a significant degree in a dense bed of relatively small volume, after which the catalyst together with effluent gases from the dense bed is passed up a riser in which combustion of the carbon monoxide takes place with a resultant, highly efficient transfer of heat from the CO oxidation reaction to the catalyst in the riser. The presence of the catalyst in the region where the CO oxidation takes place prevents damage to the regenerator equipment because the catalyst acts as a heat sink for the CO oxidation reaction and, in this way, the twin objectives of obtaining a clean-burned catalyst and of ensuring continued structural integrity of the regenerator equipment are assured. In this type of regenerator, recycle of the hot regenerated catalyst from the top of the riser to the dense bed is provided in order to provide an adequately high temperature in the dense bed for rapid combustion of the coke on the spent catalyst. Regenerators of this type are described in U.S. Pat. No. 3,926,778 and high-efficiency, low-inventory regenerator type units of this type have enjoyed a significant commercial success. A regenerator of this type is also shown in U.S. Pat. No. 4,072,600 (FIG. 4), since the use of a CO oxidation promoter is desirable for this type of regenerator as with the more conventional, high-inventory regenerator.
As with the use of zeolite cracking catalysts, however, the advantages of CO burning in the regenerator have not been bought without cost. As pointed out above, the combustion of carbon monoxide to carbon dioxide is a strongly exothermic reaction and in the high-inventory, dense type bed regenerator the release of heat from this reaction may cause significant problems if it takes place above the dense bed of catalyst. In the dilute catalyst phase above the dense bed, some catalyst is present to act as a heat sink for the released heat but even then, insufficient catalyst may be present so that the heat will be carried off in the effluent gases through the cyclones resulting in an increase in cyclone temperature, possibly to undesirably high values. If the CO oxidation front moves up to the cyclone inlets, equipment failure is highly likely as a result of the extreme exotherms in this region.
Regenerator operation in the CO oxidation mode has therefore been concerned with mitigating the effects of "afterburning", the term commonly used to designate oxidation of the carbon monoxide above the dense fluid bed. The use of combustion promoters produces a significant improvement because the promoters increase the degree of oxidation which takes place in the dense bed. However, there are limits on the amounts of promoter which may be used because high promoter levels are associated with high levels of nitrogen oxides (NO.sub.x) in the regenerator effluent gases as discussed in U.S. Pat. No. 4,235,704. Other improvements have been achieved by changes in reactor construction to improve the contact between the catalyst and the regeneration air, for example, as described in U.S. Pat. Nos. 4,118,448, 4,118,337, 4,387,043, 3,990,992 and 4,219,442.
Another problem frequently encountered, especially in large regenerators is that of poor air/catalyst contacting efficiency. This may be occasioned by bubble formation in the bed so that the gas bypasses the catalyst or uneven carbon distribution in the dense bed as a result of imperfect solids mixing. This too manifests itself by afterburning in the dilute phase where the absence of a heat sink leads to localized dilute phase hot spots and possibly to damage to the cyclones. If this problem is of a continuing nature with any unit and catalyst, it may become necessary to limit the severity of the cracking operation in order that the regenerator can be safely operated within acceptable limits. Thus, the refiner may find his cracking operation undesirably restricted by limits on the regenerator operation.
Another problem which is encountered with the regenerator operating with noble metal-promoted complete carbon monoxide combustion is that excessive amounts of nitrogen oxides (NO.sub.x) may be produced in the regenerator. The use of certain additives such as palladium and ruthenium for promoting CO combustion without causing the formation of excessive amounts of nitrogen oxides is described in U.S. Pat. Nos. 4,300,947 and 4,350,615.
Studies of fluidized bed combustion of coal have shown that staged combustion is an effective way of reducing NO.sub.x emissions if the lower part of the combustor is operated to achieve a region which is rich in gaseous, oxidizable carbon (CO and coal volatiles) and relatively depleted in oxygen. This atmosphere reduces NO.sub.x species to molecular nitrogen, ammonia and other nitrogenous compounds. Secondary air is introduced at a higher level in the combustor to complete oxidation of the oxidizable, gaseous carbon content but without causing oxidation of the reduced NO.sub.x species. Reference is made to Bergsmeyer, F. "Abatement of NO.sub.x from Coal Combustion. Chemical background and Present State of Technical Development" Ind. Eng. Chem. Process Des. Dev. 24 (1), 1985. These concepts have been extended to the staged FCC catalyst regeneration as described in U.S. Pat. Nos. 4,309,309 and 4,313,848 where multiple air distributors in the dense bed with external recirculation of regenerated catalyst are employed to achieve staged combustion. Whatever the merits of this proposal, it is undesirable from the point of view of mechanical complications as well as of equipment capital cost.